1. Field of the Invention
The present invention relates to a sheet feeding apparatus for drawing out a pile of sheets such as plain paper and recording papers, one by one, (including the case of a pile thereof in a cassette), particularly to the structure enabling to feed special types of sheets having a heavy specific gravity or more rigidity or more thickness such that the powers for drawing out is insufficient as compared with usual sheets, for example, plain papers which are used for copying or recording machine.
2. Description of the Prior Art
Usually in an electrophotographic copying machines using plain papers as transfer sheets or recording sheets or in a facsimile transmitting apparatus, the sheet feeding apparatuses having a variety of structures are being used to draw out sheets one by one.
As a matter of course, such sheet feeding apparatuses are requested to have a simple structure, few troubles and a stability in operation. Among many conventional apparatuses used, good results have been produced by the apparatuses, for instance, having a structure shown in FIG. 1 in case of no cassette having been used, and having a structure shown in FIG. 4 in case of sheets having been piled in a cassette.
Referring to FIG. 1, the reference character A represents a paper feeding table at one end of which the base portion of the sheet feeding tray B is pivotally supported by the pin C. The member opposing to the free end of said sheet feeding tray B is the wall E which is provided on the main frame side for the purpose of regulatively positioning the ends of a number of sheets D set on said sheet feeding tray. Above the end (the free end) of said sheet feeding tray B, there is the roller type sheet feeding means F positioned so that each sheet D may be drawn out one by one from the top of sheets D piled by friction force with the process in operation. And, at a part of said sheet feeding tray B, there is a biasing member, i.e., the tension spring G, suspended to keep the upper surface of the end of top sheet D down to the circumferential surface of the sheet feeding means F. In such sheet feeding apparatuses having this type of structure, the suitable friction force generated between the sheet feeding means F and the sheet D may be obtained by observing the tension of the tension spring G in experiments and then by adjusting the tension force to a fixed degree, that is needless to say. However, in the paper feeding apparatus having such a type of the structures, there have been many cases difficult to feed sheets due to the shortage of force for drawing out sheets by sheet feeding means F, when heavy specific gravitative, thick or rigid sheets, other than sheets usually used, might be tried to feed.
Next, referring now to the example using a cassette, different from the aforedescribed example.
In FIG. 4 illustrating a conventional apparatus, the reference character H represents a cassette that is detachable from the main body of an apparatus and inside of which the base portion of a middle plate J is pivotally suspended so as to be movable as shown by the Fig., and also the fixedly positioned separation claws K are fitted up in the both corners of the front end of the cassette H in order to catch the upper most sheet and to protect from double feeding of sheets as publicly known. And, above the end (the free end) of said middle plate J, the sheet feeding means F comprising a roller for drawing out each sheet D one by one from the upper most thereof, by the aid of friction force. Further, the push lever L energized by the spring M is kept in upward contact with the bottom surface of said middle plate J. Therefore, in a sheet feeding apparatus having this type of construction, the suitable friction force generated between the sheet feeding means F and the sheet D may be obtained by adjusting the elastic force of the spring M at a fixed degree. However, as described above, in a sheet feeding apparatus having such this type of construction, there have been many cases difficult to feed sheets due to the shortage of the force for drawing out sheets when it has been desired to feed sheets having a heavier specific gravity, more thickness, or rigidity in comparison with plain papers and the like normally used.